Greninja

Greninja is a threatening Pokemon in the 1v1 metagame, with a good Special Attack and an amazing Speed tier that make it very threatening, being able to OHKO many threats like Mega Swampert, Zygarde-C, and Dragonite. Backing up that statline is access to Protean with a great movepool, giving it more offensive pressure with a STAB boost on all of its moves and a way to evade powerful attacks by changing to a type that resists them or is immune to them. A Substitute + Torrent set can also be used to beat threats to the Choice Specs set like specially defensive Mega Charizard X and Y, Mega Metagross, and Type: Null. By using the increased utility from forgoing coverage moves for options like Substitute, Work Up, and Taunt, Greninja can pump out a huge boosted Hydro Cannon while still using status moves to lock down foes. However, it is horribly frail, so any super effective moves against it will likely OHKO it, and it also falls to some strong neutral STAB attacks such as Adamant Haxorus's Outrage, Mega Lopunny's Giga Impact, and Mega Gyarados's Outrage. Greninja can also be defeated by faster Pokemon that can take advantage of its low defenses like Zeraora and bulkier ones that aren't afraid to take its hits like Kyurem